全文获取类型
收费全文 | 7081篇 |
免费 | 1436篇 |
国内免费 | 1667篇 |
专业分类
测绘学 | 520篇 |
大气科学 | 1209篇 |
地球物理 | 1965篇 |
地质学 | 3669篇 |
海洋学 | 929篇 |
天文学 | 437篇 |
综合类 | 611篇 |
自然地理 | 844篇 |
出版年
2024年 | 28篇 |
2023年 | 107篇 |
2022年 | 297篇 |
2021年 | 378篇 |
2020年 | 284篇 |
2019年 | 363篇 |
2018年 | 401篇 |
2017年 | 361篇 |
2016年 | 408篇 |
2015年 | 358篇 |
2014年 | 455篇 |
2013年 | 432篇 |
2012年 | 464篇 |
2011年 | 486篇 |
2010年 | 439篇 |
2009年 | 421篇 |
2008年 | 372篇 |
2007年 | 330篇 |
2006年 | 294篇 |
2005年 | 309篇 |
2004年 | 250篇 |
2003年 | 262篇 |
2002年 | 281篇 |
2001年 | 293篇 |
2000年 | 255篇 |
1999年 | 282篇 |
1998年 | 234篇 |
1997年 | 216篇 |
1996年 | 190篇 |
1995年 | 140篇 |
1994年 | 135篇 |
1993年 | 132篇 |
1992年 | 108篇 |
1991年 | 77篇 |
1990年 | 56篇 |
1989年 | 77篇 |
1988年 | 52篇 |
1987年 | 33篇 |
1986年 | 29篇 |
1985年 | 27篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1975年 | 2篇 |
1958年 | 6篇 |
1954年 | 3篇 |
1942年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Taking into account magneto-optical effects, we have obtained numerical solutions of the transfer equations for the Stokes parameters, calculated the linearly polarized intensity (U) and constructed its monochromatic images of unipolar sunspots. By comparison with the observational material of the vector magnetograph of the Marshall Space Flight Center, Huntsville (Alabama), we have found that the model of radial magnetic fields may give rise to U monochromatic images close to those observed. The same conclusion has been obtained previously by Landi Degl'Innocenti (1979), although his analysis was performed with the Milne-Eddington approximation instead of a detailed sunspot model. Moreover, we have shown that the model of spiral magnetic fields leads to results in contrast with observations. 相似文献
22.
23.
煤岩对压裂裂缝长期导流能力影响的实验研究 总被引:7,自引:0,他引:7
煤层中一般都伴有煤层气,通过压裂可以将煤层气释放出来,一方面能够获得清洁能源,另一方面可以提高煤田的安全开采。煤岩层自身特点决定了其在压裂中应用的实验评价方法及结论应具有特殊性,本文主要针对煤层气井的煤岩层压裂裂缝长期导流能力进行实验评价研究。实验表明,与砂岩地层不同,煤岩的硬度较小,压裂中支撑剂嵌入情况较严重;煤岩易破碎,碎屑颗粒充填到支撑剂中;同时支撑剂存在破碎及吸附伤害等,多种作用导致支撑裂缝的导流能力降低。实验中,以煤岩层闭合压力为一个重要的参数,实验对比分析了在不同闭合压力下,铺砂浓度、支撑剂粒径、支撑剂种类及组合的选择、支撑剂颗粒的破碎与嵌入、压裂液残渣等因素对煤岩层导流能力的影响。本文的实验结论为,嵌入对煤岩层支撑裂缝的导流能力伤害很大,加大铺砂浓度、采用低破碎率支撑剂、降低压裂液伤害能在一定程度上提高煤层裂缝的导流能力,但影响程度随闭合压力的增加而逐渐降低。本文所得出的结论对今后煤层气的开发及设计施工具有一定的指导意义。 相似文献
24.
Huijun Jin Zhi Wei Shaoling Wang Qihao Yu Lanzhi Lü Qingbai Wu Yanjun Ji 《Engineering Geology》2008,101(3-4):96-109
The Qinghai–Tibet Highway and Railway (the Corridor) across the Qinghai–Tibet Plateau traverses 670 km of permafrost and seasonally frozen-ground in the interior of the Plateau, which is sensitive to climatic and anthropogenic environmental changes. The frozen-ground conditions for engineering geology along the Corridor is complicated by the variability in the near-surface lithology, and the mosaic presence of warm permafrost and talik in a periglacial environment. Differential settlement is the major frost-effect problem encountered over permafrost areas. The traditional classification of frozen ground based on the areal distribution of permafrost is too generalized for engineering purposes and a more refined classification is necessary for engineering design and construction. A proposed classification of 51 zones, sub-zones, and sections of frozen ground has been widely adopted for the design and construction of foundations in the portion of the Corridor studied. The mean annual ground temperature (MAGT), near-surface soil types and moisture content, and active faults and topography are most commonly the primary controlling factors in this classification. However, other factors, such as local microreliefs, drainage conditions, and snow and vegetation covers also exert important influences on the features of frozen ground. About 60% of the total length of the Corridor studied possesses reasonably good frozen-ground conditions, which do not need special mitigative measures for frost hazards. However, other sections, such as warm and ice-rich or -saturated permafrost, particularly in the sections in wetlands, ground improvement measures such as elevated land bridges and passive or proactive cooling techniques need to be applied to ensure the long-term stability of thermally unstable, thick permafrost subsoils, and/or refill with non-frost-susceptible soils. Due to the long-history of the construction and management of the Corridor by various government departments, adverse impacts of construction and operation on the permafrost environment have been resulted. It is recommended that an integrated, executable plan for the routing of major construction projects within this transportation corridor be established and long-term monitoring networks installed for evaluating and mitigating the impact from anthropogenic and climatic changes in frozen-ground conditions. 相似文献
25.
The concentrations of twenty four chemical elements in the surface layer of natural desert soils and the cultivated farmland soils were measured at a desert-oasis ecotone in the middle of Heihe river basin, north-west China. Background values were estimated for (a) major elements (Si 335.3 g kg− 1, Al 49.4 g kg− 1, Fe 19.1 g kg− 1, Ca 29.4 g kg− 1, Mg 8.9 g kg− 1, K 20.1 g kg− 1, Na 17.5 g kg− 1 and P 0.338 g kg− 1), (b) heavy metals and non-metals (Cr 55.8 mg kg− 1, Mn 404.8 mg kg− 1, Ni 17.7 mg kg− 1, Cu 5.1 mg kg− 1, Zn 33.7 mg kg− 1, Pb 15.5 mg kg− 1 and As 5.2 mg kg− 1) and (c) other trace elements (Ti 2.0 mg kg− 1, V 55.3 mg kg− 1, Co 5.7 mg kg− 1, Rb 82.4 mg kg− 1, Sr 232.9 mg kg− 1, Y 14.7 mg kg− 1, Zr 194.9 mg kg− 1, Nb 7.8 mg kg− 1 and Ba 720.6 mg kg− 1). After natural desert soil was cultivated for agricultural use, significant changes in element concentrations occurred under tillage, irrigation and fertilisation management. Compared to natural soil, the for the levels of Si, K, Na, Sr, Zr and Ba decreased, and no changes were observed for Rb, while the values of the other 17 elements increase in agricultural soil from 1.2 to 3.5 times. However, their absolute concentrations are still low, suggesting that the arable soil in this region remains comparatively a clean soil. The increased silt, clay and organic carbon content, under long-term irrigation, enriched the fine-grained materials, and application of fertilisers and manure contributed to the accumulation of most elements in arable soil. The accumulation of elements in agricultural soil increased with increasing cultivation years and extent of soil development. 相似文献
26.
西藏冈底斯带措勤地区则弄群火山岩锆石U-Pb年代学格架及构造意义 总被引:29,自引:12,他引:17
大规模的则弄群火山岩呈带状近东西向展布于西藏冈底斯带中北部地区.对出露状况较好的措勤地区则弄群火山岩进行了锆石U-Pb定年,以建立其年代学格架便于区域对比.措勤地区则弄群火山岩多数锆石具有生长振荡环带,部分锆石显示核边结构.在措勤达雄北西部上覆于中二叠统下拉组灰岩的1件则弄群英安岩样品的LA-ICP-MS锆石U-Ph年龄为130±1Ma,1件则弄群流纹岩样品的SHRIMP锆石U-Pb年龄为129±3Ma;在措勤达雄北东部与下白垩统多尼组碎屑岩呈断层接触的1件则弄群英安岩样品中的锆石具有两组LA-ICP-MS锆石U-Pb年龄,最年轻的一组(包括增生边)为121±1Ma,较老的一组(包括核部)为131±1Ma;在措勤南西部1件则异群流纹岩样品的LA-ICP-MS锆石U-Pb年龄为111±1Ma,与措勤地区花岗岩类的侵位时代相当.这些高质量的锆石U-Pb年代学数据表明,措勤地区的则弄群火山作用很可能开始于约130Ma,停息于约110Ma,持续时间约20Ma.区域对比表明,东西延伸约1000km的则弄群火山作用同时发生在约130Ma.区域上目前的年代学数据结合同时代岩浆作用的分布特征表明,冈底斯带中北部地区早白垩世岩浆作用不太可能由新特提斯洋壳向北的低角度或平板俯冲产生. 相似文献
27.
乌鲁木齐河流域地下水水质监测网设计 总被引:1,自引:0,他引:1
文章运用地下水易污性编图及污染源分布图法进行了乌鲁木齐河流域地下水水质监测网设计。共设计了130监测孔,现有46个监测孔,另需要84个新的监测孔。按监测类型分为面源监测点22个,点源监测点87个,重点水源地与泉水监测点21个。按监测运行分长期监测点55个,流域普查监测点75个。普查监测点监测频率为1次/5年,长期监测点监测频率为1次/年。首期有针对性地在污染严重的柴窝堡新化厂排污区、乌鲁木齐河谷老排污区、米泉污灌区、米泉工业污染区、老龙河污染区取了25个污染水样测试分析,结果显示地下水已经受到严重污染。 相似文献
28.
旋转轴套在超深井工作时,其井下流动是三维湍流流动,旋转轴套的旋转和表面曲率效应以及随之而来的哥氏力和离心力,使流场在超深井的流动中极其复杂,同时,由于流体介质泥浆属于液固两相流,更致使内部流场测试困难,而且超深井中的工况在使用常规方法已很难得到较准确的数据。为此,将计算流体力学软件Fluent应用于超深井下流场的模拟,基于Navier-Stokes方程和Reynolds应力方程模型,建立多种仿真模型,在相同条件下,使用CFD仿真软件Fluent模拟仿真的不同结果,优化与旋转轴套叶片设计相关的几何参数,提高 相似文献
29.
30.